Chemical Engineering February 2021 - 42

extent (nonhazardous area).
If you can design and install a ventiFIGURE
3. Fixed gas detectors can be an important
mitigation measure
some operations and apparatus is
so infrequent that area classification
is not necessary. For example, it is
not usually necessary to classify the
following locations where combustible
materials are processed, stored,
or handled:
(1) Locations
that have adequate
ventilation, where combustible materials
are contained within suitable, well
maintained, closed piping systems
(2) Locations that lack adequate ventilation,
but where piping systems
are without valves, fittings, flanges
and similar accessories that may be
prone to leaks
(3) Locations where combustible materials
are stored in suitable containers
(4) Locations where the use of combustible
liquids, or flammable liquids
or gases, will not produce gas or
vapor sufficient to reach 25 percent
of the lower flammable limit (LFL) of
that combustible material
A very important concept if one
chooses to use ventilation as a mitigative
measure is keeping the ignitable
area to a negligible extent. ISA
12.24.01 Recommended Practice
for Classification of Locations for
Electrical Installations Classified as
Class I, Zone 0, Zone 1, or Zone 2
gave excellent guidance in this regard
in 5.3:
The most important factor is that
the degree or amount of ventilation
is directly related to the types
of sources of release and their corresponding
release rates. This is irrespective
of the type of ventilation,
whether it be wind speed or the
number of air changes per time unit.
Thus, optimal ventilation conditions in
the hazardous area can be achieved,
and the higher the amount of ventilation
in respect of the possible release
rates, the smaller will be the extent of
the zones (hazardous areas), in some
cases reducing them to a negligible
42
lation system so that it will capture all
the flammable or combustible material
effectively at the source, then you
should not need to classify an area.
Consider an open drum full of
flammable material. For some distance
above the drum, an ignitable
mixture of flammable vapor and air
(that is, a mixture over the LEL) is
present. Classifying this area in lieu
of other preventive measures is only
prudent. Alternately, if one were to
install a properly designed ventilation
system that drew off these vapors
immediately at the surface of the
drum then the extent of this ignitable
area would be very small; in the code
language a " negligible extent " . Suppose
the design is effective in capturing
all the vapors within one inch
off the top of the drum. Does classifying
the area several feet above
the top of the drum and off to the
side make any sense? At worst one
might consider this one inch area
above the drum as Class I Division 1
but then classify the rest of the area
as unclassified. (And prudently don't
put any electrical devices or wiring in
that one inch.)
Ventilation as a mitigative measure
requires careful design, proper
installation and testing to confirm it
is effective, regular maintenance to
keep it working properly, and appropriate
failsafe alarms and safeguards
to warn of its failure and take the
proper actions to keep the area safe.
All these are required. They also cost
and require routine attention.
Simply doing a calculation that indicates
that the ventilation rate will
dilute the flammable or combustible
material below its LEL is not enough
for many reasons. First, the larger
the area within the explosive range,
the higher the change of encountering
an ignition source before dispersion.
So, there is a real need to
design a system to keep this extent
small, approaching negligible.
Second, theoretical gas dispersions
doe not approach real world
conditions. American Conference of
Governmental Industrial Hygienists'
(ACGIH) " Industrial Ventilation, A
Manual of Recommended Practice
for Design " suggests a mixing factor
to account for imperfect mixing that
ranges from 1 to 10. Other sources
suggest a similar safety factor of 2 to
5. Regardless of what value is used,
the key point to recognize is that theoretical
calculations rarely approach
real-world conditions. Even computational
fluid dynamics (CFD) simulations
will invariably prove to be less
accurate than expected.
Third, pressurized leak sources
travel significantly longer distances.
API 500 in Table 1 suggests an adjustment
factor of 1.5 for pressures
above 741 psig and 2.5 for pressures
above 1,440 psig. Gas leaks
will stay concentrated for significant
distances before slowing and beginning
to disperse. Finally, all ventilation
calculations require you to assume a
maximum leak rate. This is an area
fraught with risk as too many things
affect the size of a leak. Hence it can
often be much larger or in a different
place than expected.
The only truly safe approach is to
either enclose the potential source of
the release and ventilate it above the
maximum possible leak rate one can
credibly determine or provide a large
enough system, properly designed,
to capture the leak effectively at the
source. In both cases, by moving the
ventilation close to the source, one
keeps the area of concern negligible.
Prohibiting electrical equipment and
wiring in this limited area is usually
very feasible.
Gas monitoring. A third potential
mitigative measure is continuous
fixed gas detection (Figure 3). NFPA
70 in 500.7 states " A combustible
gas detection system shall be permitted
as a means of protection in
industrial establishments with restricted
public access and where
the conditions of maintenance and
supervision ensure that only qualified
persons service the installation. "
However, it limits use of a gas detection
system to three specific cases:
* A location that would be Class I
Division 2 based on a hazard analysis
and risk assessment except
for inadequate ventilation can, with
a fixed gas detection system, be
considered Division 2.
* A building that would not be classified
but is, wholly or partially,
inside a Division 2 area can, with
a fixed gas detection system, be
considered unclassified.
* The interior of a control cabinet inside
a Class 1 Division 1 area can,
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